Stents with the capability of drug elution can address different problems associated with coronary or peripheral vascular blockages. Conventionally, the drug is contained in a polymer coating which covers the stent and can be released from this polymer coating after the stent has been placed into the subject to be treated. Generally, conventional drug-containing coatings are for example liable to delaminations, especially during stent expansion. However, in case of a delamination of the drug-containing coating, the drug cannot be delivered to the desired location or in the desired amount. Therefore, it is important for such drug containing coatings to be able to maintain their mechanical integrity during storage and especially after stent expansion using a balloon.
Conventionally coated metal stents generally use a primer coating, typically a parylene coating, prior to the coating with bio-degradable polymer combinations in order to avoid delaminations of the coating during stent expansion. Such a primer coating is deemed essential to prevent delamination of the bio-degradable polymer coating from the metal surface, both during storage and upon balloon expansion. In cases where the polymer coating comprises a drug component, delaminations are more likely. Thus, a primer coating is generally used in order to eliminate the problems with mechanical integrity, but the primer coatings such as the parylene coatings which are generally used are, however, non-biodegradable and increase the overall coating thickness because the primer coating generally does not accommodate any drug.
Stents with poly(lactic-co-glycolic acid) (PLGA) polymer coatings have recently been described by Wormuth et al. for preventing arteries from collapsing. These coatings are capable of releasing a drug, e.g. an antiproliferative drug such as rapamycin (Wormuth et al. in Microsc. Microanal. 13 (Suppl. 2), 2008, 1690-1691). The polymer coating is based on a biodegradable polymer made of poly(lactic-co-glycolic acid) (PLGA) and comprises the drug in a concentration in the range from 5 to 50 wt %. Alternative drug-loaded polylactic acid (PLA)/polycaprolactone (PCL) polymer coatings are known to have drawbacks such as a limited thickness of the coating, which is not able to sustain the drug release over a long period, and neither is it able to control the release to any great extent. Drug loading in these coatings need to be low in order to avoid adverse embrittlement and delamination of the biodegradable polymer coating from the metal substrate.
On that basis it was recently suggested to solve the above problems by using fully bio-degradable stents which have a potential for carrying and delivering much higher drug loadings. In this case, the metal stent was replaced by a polymer stent in order to eliminate the delamination drawbacks of the conventional polymer coated non-biodegradable stents. Since in polymer stents, no primer layer is necessary anymore, the polymer coating can be made thicker compared to the polymer coating used with metal stents and, thus, a higher drug loading is possible (Wang et al. in Biomaterials 27 (2006), 5588-5595). However, such stents can only be used in temporary applications due to the fully biodegradability of the polymeric stent material and the polymer coating.
In view of the above it is desirable to have drug loaded stents that are able to sustain the drug release over a long period and/or that are able to control the release of drugs.